In recent years, as disclosed for example in the Japanese patent publications filed by this applicant and entitled "Veneer Lathe" (Japanese Patent Publication No. 116729/1981), "Veneer Lathe" (Japanese Patent Publication No. 19007/1984), and "Peripheral Driving Veneer Lathe" (Japanese Patent Publication No. 28444/1984), there is in practical use a peripheral driving veneer lathe comprising a driving roller which can be engaged with the periphery of a bolt immediately before lathing and having a structure whereby at least part of the driving force required for lathing the bolt can be supplied to the bolt at its periphery through said driving roller. With this type of veneer lathe, it becomes possible to prevent or significantly reduce problems such as breakage of the bolt, and stuffing of material at the tip of the cutting tool, both of which tend to occur in a conventional veneer lathe where the driving force is supplied to the axial duramen of the bolt exclusively through the spindles. Thus it becomes easier to handle low quality bolts, the use of which has been avoided in lathing hitherto.
Furthermore, as disclosed, for example, in "Veneer Lathe" (Japanese Patent Publication No. 21808/1986) a technique is developed to stabilize the engagement degree of a driving roller with a bolt in such a manner thqt the driving roller is displaced following the changes in the diameter of a bolt. However, even an improvement such as this in the technique is insufficient to deal appropriately with the different properties from one bolt to another, or the difference in hardness of a bolt at its alburnum and axial duramen part.
Yet another technique attempts to optimize the degree of engagement by providing a plurality of driving rollers at specific places in such a fashion that the driving rollers are pressurized by a uniform selected pressure in the centripetal direction, said pressure being adjusted in accordance with the overall characteristics of a bolt. A mechanism according to this technique was proposed in the previous application "Peripheral Driving Veneer Lathe" (Utility model application No. 54847/1989). The mechanism simply increases or decreases the pressure applied to each of the driving rollers uniformly at a plurality of specific places. A disadvantage which occasionally is encountered with the mechanism is that the engagement degree of the driving roller with the bolt deviates.
From the viewpoint of simplifying the system as a whole including the apparatus and devices required for its driving, it is desirable to form a single roller with an engagement length corresponding to the maximum length of a bolt according to specification, and apply pressure in a state where the driving roller is held only at two places at both ends thereof. However, even if the pressures are applied equally at said two places in operating such system, the engagement degree of the driving roller with the bolt deviates when the holding position for the bolt is imbalanced toward either one of the two ends as, for example, in the case of using a four foot veneer lathe for lathing a three foot bolt set with its end aligned with an end of the lathe.
Also, in practice, the driving roller must be arranged in the vicinity of the tip of the tool, and the size of the driving roller that can be mounted is limited. Consequently, the rigidity of the driving roller is also limited. As a result, where a single driving roller is pressurized only at two places at both ends thereof, a deflection tends to occur in the middle section between them. Therefore, in order to equalize the engagement degree of the driving roller with the bolt when lathing a long bolt within a practicably allowable limit, it is desirable to either apply pressures at appropriate places in the middle section in addition to the two places at both ends of the single roller, or to divide the driving roller into plural rollers, with each of the small rollers pressurized at its appropriate places. In the latter case, it is still desirable to make the number of the divided rollers as small as possible from the viewpoint of simplifying the system.
In the configuration mentioned above, the engagement degree of the driving roller with the bolt deviates if the holding position of the bolt is lopsided as in the previous case. Furthermore, even if a bolt is held with the center of a veneer lathe as its reference point to keep the balance of the holding positions for the bolt, deviation in the engagement degree still occurs when the length of the bolt changes as, for example, in the case of lathing a six foot bolt by an eight foot veneer lathe with the center thereof as the reference point for holding.
In this connection, when a longest standard bolt is lathed using a single roller with pressures applied at three places, two at both ends and one in the middle, the desirable pressure ratio at each pressurized place is approximately 1:10/3:1. In a different example in which a driving roller is divided into two parts at its center and pressurized at four places, two at both ends and two at the central parts, for example, the desirable pressure ratio is 1:1:1:1. Whatever the case may be, it is readily understandable that if a bolt shorter than the longest bolt that can be lathed is set on the basis of such ratio, the engagement degree in the vicinity of both ends of said bolt becomes relatively excessive as compared with that in the central part thereof.
The aforementioned prior art publications also disclose a driving roller comprising many spiky members mounted on its periphery, which are impaled into a bolt for engagement. This is more practicable because it is more capable of transferring the driving force as compared with a driving roller whose periphery is smooth. However, depending on the hardness of a bolt, the impaling degree of the spiky members differs, absent any modification. Hence, if this driving roller is used for lathing a bolt having hard knots locally concentrated, such as pine and the like, the engagement degree may deviate from one point to another.
The various deviations of the engagement degree such as set forth above lead to the inferior quality of a veneer as a finished product or other problems such as the breakage of a bolt. It is desired to prevent or reduce such deviation.
The purpose of the present invention, therefore, is to obtain a pressure method for preventing or reducing the deviation of the engagement degree of a driving roller with a bolt by adjustment of the pressure ratio at the plural places where the pressures are applied to the driving roller for a peripheral driving veneer lathe, so that the various adverse effects detailed above can be reduced.